Two Pointers — DSA Pattern

The Two Pointers pattern applies when you need fast-slow, converging, expanding from center, and in-place modification techniques. Its time complexity is O(n) for a single linear pass; O(n²) for centre-expansion variants (e.g. palindrome checks) and space complexity O(1). It is used in 34 problems on Thita, including Backspace String Compare, Boats to Save People and Container With Most Water. Common variations are String Comparison with Backspaces and String Reversal.

Master the two pointers technique including fast-slow pointers, converging pointers, and expanding from center. Essential for array, string, and linked list problems in coding interviews.

Two Pointers is pattern 1 of 16 in the Thita.ai DSA course. It covers fast-slow, converging, expanding from center, and in-place modification techniques, split into 7 subpatterns and about 3 hours of study. It is one of the interview-critical patterns, so it is worth covering before your first onsite.

Subpatterns of Two Pointers

Each subpattern below has its own theory page and problem set. They are listed in the order the course teaches them.

• Fast & Slow (Cycle Detection) — Master Floyd's tortoise and hare algorithm for cycle detection. Detect cycles in linked lists and find duplicate numbers. (core technique)
• Converging (Sorted Array Target Sum) — Learn converging two pointers for sorted arrays. Solve two sum, three sum, and container problems efficiently. (core technique)
• Expanding From Center (Palindromes) — Master the expand from center technique for finding palindromic substrings and longest palindrome problems. (core technique)
• In-place Array Modification — Learn in-place array modification using two pointers. Remove duplicates, move zeroes, and sort colors efficiently. (core technique)
• String Comparison with Backspaces — Learn to compare strings with backspace characters using two pointers from the end. (important variation)
• String Reversal — Master string reversal techniques including reverse string, reverse words, and reverse vowels using two pointers. (important variation)
• Fixed Separation (Nth Node from End) — Master fixed separation two pointers to find nth node from end, middle of list, and similar problems. (important variation)

How to practise Two Pointers on Thita.ai

Read the theory for a subpattern, then solve its problems in the browser editor. Your solution runs against the problem's test cases, and the AI coach gives a hint on the technique you are missing rather than the finished answer. Progress is tracked per subpattern, so the tracker shows which of the 7 Two Pointers subpatterns you have covered. Topics covered here include two pointers, two pointer technique, fast slow pointers, converging pointers, floyd cycle detection, two sum, three sum, palindrome check.

Related DSA patterns

• Sliding Window — Fixed-size, variable-size, and character frequency matching window techniques.
• Binary Search Patterns — Standard binary search, rotated arrays, first/last occurrence, and search on answer space.
• Array/Matrix Manipulation Patterns — In-place operations, hashing, prefix sums, spiral traversal, and cyclic sort.
• Linked List Manipulation Patterns — In-place reversal, merging sorted lists, reordering, and intersection detection.
• Stack Patterns — Parentheses matching, monotonic stack, histogram, and expression evaluation.
• All 16 DSA patterns
• The DSA patterns sheet
• Coding practice